In this paper solutions are presented in terms of (double) Wronskians.
Solutions in this form and similar forms are usually obtained via Darboux transformations
or bilinear method.
Compared with other popular forms of \(N\) -soliton solutions (e.g. Hirota's form using polynomials of
exponential functions given in Appendix 
and dressed Cauchy matrix form obtained in [1]}, [2]}, [3]}),
by virtue of their special structure,
Wronskian solutions have advantage in presenting limit solutions, i.e. multiple-pole solutions,
In bilinear approach, such solutions are alternatively obtained by taking, for example,
\(A\)  in (REF ) to be composed of Jordan blocks.
One may refer to [4]} (see page 22 of the book) for the limit procedure
and to [5]} for the connections between the LTTMs and limit solutions.
For the multiple-pole solutions in terms of dressed Cauchy matrix form, one may refer to
[6]} for the Riemann-Hilbert method and to [7]}
for the Cauchy matrix approach.
In addition, by employing double Wronskians, coefficient matrix \(A\)  and bilinear approach,
we have also illustrated an effective reduction technique to obtain solutions for the reduced equations.
In this technique, looking for vectors \(\phi \)  and \(\psi \)  such that \(u\)  and \(v\)  satisfy desired constraints
when they are expressed in terms of double Wronskians \(f,g,h\)  and \(s\) , is boiled down to solving the
algebraic equation (REF ) (and (REF ) for nonlocal case).
This enables us to approach to new solutions that might be missed before,
for example, the case (2) in Table REF  that corresponds to real eigenvalues and the mixed case related to
(REF ).
